Salmonellosis continues to be a major infectious disease in both the United States and elsewhere. The overall goal of this project is to gain a better understanding of the early events that occur during Salmonella infections. The proposed studies focus on the initial interactions of salmonellae with host defense cells, specifically neutrophils and macrophages. The first objective is to determine whether structures found on the outer surface of Salmonella, known as porins, are involved in the recognition of this pathogen by human neutrophils. Porin-deficient mutants will be compared with their corresponding wildtype counterparts in their ability to adhere to and be internalized and killed by these neutrophils. Microbial attachment will be measured using flow cytometry and fluorescence microscopy and internalization and killing, by viability assays. The second objective is to determine whether neutrophils that have passed across a model intestinal epithelial cell layer are modified in their ability to recognize and to kill Salmonella. A model has been developed in which Salmonella initiate a series of events in the intestine that result in the migration of neutrophils into the lumen. The goal of this study is to determine whether these neutrophils exhibit an enhanced ability to detect and kill these bacterial pathogens. The third objective focuses on the ability of purified porins to block the attachment of Salmonella to host defense cells. Highly purified porins and porin-lipopolysaccharide complexes will be used in in vitro competition studies. Taken together, these studies are expected to provide a better understanding of the early cellular events in Salmonella infections.